R-T-B type magnets, which have the highest magnetic energy product among permanent magnets, have been applied to hard disks (HD), MRIs (magnetic resonance imaging), various types of motors and the like due to their excellent properties. In recent years, their application for motors used in cars has been increased because energy saving is highly expected and the heat-resistance of the R-T-B type magnets has been improved.
The R-T-B type magnets mainly contain Nd, Fe, and B, and therefore, they are generally called an “Nd—Fe—B type” or “R-T-B type” magnet. The R of the R-T-B type magnet represents mainly those in which a part of Nd is substituted with other rare earth elements such as Pr, Dy, and Tb, namely at least one of these rare earth elements including Y T represents those in which a part of Fe is substituted with metals such as Co and Ni. B represents Boron in which a part of Boron can be substituted with C or N. In addition, Cu, Al, Ti, V, Cr, Ga, Mn, Nb, Ta, Mo, W, Ca, Sn, Zr, Hf, etc. may be added singularly or in combination as additional elements in the R-T-B type magnet.
An R-T-B type alloy, which turns into the R-T-B type magnet, is an alloy which has a main phase of R2T14B, namely a ferromagnetic phase that contributes to magnetization, and which simultaneously has non-magnetic R-rich phase having a low melting temperature in which the rare-earth elements are concentrated, and the R-T-B type alloy is an active metal. Therefore, the R-T-B type alloy has been molten or cast generally in a vacuum or an inert gas. Also, in order to produce a sintered magnet from a block of the cast R-T-B type alloy by way of the powdered metal technique, the block of the alloy is crushed into alloy powder of about 3 μm (measured using a Fisher Sub-Sieve Sizer (FSSS)), then pressed in a magnetic field, and sintered at a high temperature of about 1000° C. to 1100° C. in a sintering furnace. Then, the sintered alloy is generally subjected to the heating treatment, mechanical processing, and further plated to improve the erosion resistance, thereby forming a sintered magnet.
The R-rich phase of the R-T-B type sintered magnet has the following important roles:    1) The R-rich phase has a low melting temperature, becomes a liquid phase when sintered, and contributes to the densification of the magnet, namely the improvement of the magnetization;    2) The R-rich phase eliminates irregularities of the grain boundary, reduces nucleation sites in the reverse magnetic domain, and enhances the coercive force; and    3) The magnetic R-rich phase magnetically insulates the main phase, and increases the coercive force.
Consequently, if the state of the dispersion of the R-rich phases in the shaped magnet is inferior, this causes partial sintering-deficiency and low magnetization, and it is therefore important that the R-rich phases are uniformly dispersed in the shaped magnet. The distribution of the R-rich phases is considerably affected by the organization of the material, namely R-T-B type alloy.
Another problem rises in the casting of the R-T-B type alloy, in which α-Fe generates in the cast alloy. α-Fe has deformability, and therefore, is not crushed and remains in the crusher. This not only decreases the crushing efficiency of the alloy but also affects compositional alteration and the particle size distribution before and after crushing. Moreover, if α-Fe remains in the magnet even after sintering, then the magnetic property of the magnet will deteriorate. Therefore, it has been considered that α-Fe should be excluded from the material alloy as much as possible. This is why α-Fe has been eliminated in conventional alloys by subjecting them to homogenizing treatment conducted at a high temperature for an extended time where necessary. A small amount of α-Fe present in the material alloy can be eliminated by way of the homogenizing treatment. However, the elimination requires the solid phase dispersion at an extended period because α-Fe is present as peritectic nuclei. Consequently, the elimination of α-Fe is actually extremely difficult when the ingots have a thickness of several centimeters and the amount of the rare earth elements is 33% or less.
The strip cast method (abbreviated as “SC method”) has been developed and applied to practical processes, in which a block of the alloy is cast at a more rapid cooling rate in order to solve the problem in which α-Fe generates in the R-T-B type alloy.
The SC method is a technique in which a thin lamina of about 0.1 mm to 1 mm is cast by pouring the molten alloy onto a copper roller whose inside is water-cooled, and the alloy is quenched and solidified. In the SC method, because the molten alloy is extensively cooled to the temperature at which an R2T14B phase (main phase) generates, or below this temperature, the R2T14B phase can be generated directly from the molten alloy and the deposition of α-Fe can be controlled. Furthermore, the crystalline organization of the alloy becomes refined by way of the SC method, and therefore, an alloy that has an organization where the R-rich phases are finely dispersed can be produced. The R-rich phase reacts with hydrogen in a hydrogen atmosphere, expands and becomes a fragile hydride. By applying this property, fine cracks are incorporated therein, matching the degree of the dispersion of the R-rich phases. When the alloy is finely crushed after such a hydrogenation process, a large number of the fine cracks generated by way of the hydrogenation causes the alloy to break, and the crushability is very excellent. Thus, as a thin lamina of the alloy cast by way of the SC method has internal R-rich phases finely dispersed therein, the dispersibility of the R-rich phases in the crushed and sintered magnet is also excellent, thereby successfully improving the magnetic property of the magnet (for example, patent document 1).
In addition, the thin lamina of the alloy cast by way of the SC method has excellent homogeneity of the organization. The homogeneity of the organization can be compared with respect to particle diameters of the crystals or the state of the dispersion of the Rich-phases. In the thin lamina of the alloy produced by way of the SC method, although chill crystals sometimes generate on the side of the thin lamina adjacent to the casting roller (hereinafter, referred as the “casting-roller side”), a properly refined and homogeneous organization on the whole, which resulted from the rapid-cooling and solidification, can be obtained.
As explained above, when the R-T-B type alloy cast by way of the SC method is applied to the production of a sintered magnet, the homogeneity of the R-rich phases in the produced magnet is enhanced, and the harmful effects to the crushing process and the magnetization owing to α-Fe can also be prevented. Thus, the block of the R-T-B type alloy cast by way of the SC method has an excellent organization for producing a sintered magnet. However, as the properties of the magnet improve, further improvement of the R-T-B type alloy has been sought.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H5-222488